1. Field of the Invention
The field of the invention is electrochemical cells having electrodes and an absorber-separator wherein at least one of the electrodes or the absorber-separator is constructed of a porous synthetic polymer, the electrode materials having electrode material combined therewith and the absorber-separator having an electrolyte combined therewith.
2. Description of Related Art
Guyomard and Tarascon, in an article "Rocking-Chair or Lithium-Ion Rechargeable Lithium Batteries," Adv. Mater. 1994, 6, No. 5, pp. 408-12, describe a new lithium polymer electrolyte rechargeable cell and review the recent advances in the field of Li-ion rechargeable batteries.
Early lithium batteries used an intercalation material as a positive electrode which had the ability to reversibly incorporate lithium ions in its structure. The intercalation material sometimes is referred to as a "lithium sponge." The intercalation material was employed as an anode whereas the cathode consisted of lithium metal, the two electrodes being separated by a conductive electrolyte. The intercalation material consisted of layered chalcogenides such as titanium disulfide but, more recently, oxides have been studied which allow higher operating voltages and higher specific energies.
Although primary lithium cells have been commercialized, secondary lithium cells have encountered problems arising from the use of lithium metal and a liquid organic electrolyte primarily because of dendritic regrowth of lithium on the anode upon cycling which short circuits the cell. Elimination of the problem associated with lithium metal dendritic growth is now possible by employing a material able to intercalate lithium ions reversibly at very low voltages, leading to the so-called "lithium-ion," "rocking-chair," or "swing" lithium rechargeable batteries. These lithium cells operate on the principle that they contain not lithium metal, but lithium ions which are rocked back and forth between two intercalation materials (the two lithium sponges) during the charging and discharging parts of the cycle. One electrode material intercalates lithium ions, the positive during discharge and the negative during charge, while the other one deintercalates lithium at the same time. Accordingly, the lithium ion that cycles in the cell must be initially present in the structure of one of the electrode materials.
The rocking-chair approach has been possible only since about 1990 because of the previous lack of suitable reversible negative electrode materials. It was only after the discovery of some forms of carbon as lithium reversible intercalation materials that lead to the employment of carbon LiCoO.sub.2 by Sony Energytec Inc. and carbon LiNiO.sub.2 by Moli Energy Ltd. Bellcore also developed a rechargeable battery based on carbon Li.sub.x Mn.sub.2 O.sub.4 at about the same time. See, Tarascon and Guyomard, J. Electrochem. Soc. 1991, 138, 2864.
The rocking chair battery can be represented in terms of the following chain: M.sub.1 /composite positive electrode (CPE)/electrolyte (El)/composite negative electrode (CNE)/M.sub.2, where CPE and CNE are a mixture of the active material, carbon black and an organic binder, El is an electrolyte consisting of a mixture of several organic solvents and one or several lithium salts, and M.sub.1 and M.sub.2 are the external current collectors for the positive and the negative electrodes respectively. Complete chemical and electrochemical compatibility between all the elements in the chain is required.
Each half-cell, i.e., M.sub.1 CPE El and M.sub.2 /CNE El, first has to be optimized against a pure lithium metal electrode, which acts as a reference (constant voltage) electrode. In the final rocking-chair cell, the mass of the positive and negative electrode materials has to be balanced to give the same capacity for lithium deintercalation from the CPE and lithium intercalation in the CNE processes that occur at the same time when the rocking-chair cell is being charged.
One of the difficulties that occurred in rocking-chair cells was electrolyte oxidation during the charge cycle which became a more serious problem with increasing operating temperatures. Electrolyte oxidation leads to irreversible losses in capacity because of the generation of chemical species that deposit as an insulating layer on the electrode surface or evolve as a gas, thus increasing the internal pressure in the cell. Electrolyte oxidation is the main failure mechanism for this cell technology.
Bellcore recently developed a series of new ethylene carbonate (EC), dimethyl-carbonate (DMC), LiPF.sub.6 -based electrolyte compositions that are stable against oxidation and high operating temperatures and have been effectively used in a cell containing a carbon Li.sub.x Mn.sub.2 O.sub.4 electrode, where x is 1 or 2. Other electrolytes that have been developed in this regard include LiAsF.sub.6, LiClO.sub.4, LiN(CF.sub.3 SO.sub.2)2, LiPF.sub.4, LiCF.sub.3 SO.sub.3 and LiSbF.sub.6. It was found that these electrolytes and especially LiPF.sub.6 are compatible with carbon LiNiO.sub.2 and carbon LiCoO.sub.2 electrode materials as well in rocking-chair cell applications.
Prior to the introduction of carbon Li.sub.x Mn.sub.2 O.sub.4, it was found that the time needed to discharge the rocking-chair cell is shorter than the charge time by a factor of about 25% which is due to the carbon electrode where electrons are irreversibly consumed during the first lithium intercalation by side reactions at the surface of the carbon grains. About 25% of the total lithium is trapped in a superficial layer and cannot be cycled in the cell anymore. It was found that LiMn.sub.2 O.sub.4 material intercalates reversibly one extra lithium per formula unit, leading to the composition Li.sub.2 Mn.sub.2 O.sub.4. Thus it can be seen in the foregoing formula that x has a value of 1 or 2, and the excess lithium in the permanganate is used to compensate exactly the capacity lost on carbon during the first charge of the cell. This principle of the use of an extra lithium reservoir results in an increase of the specific capacity and energy of the system by about 10% which is not possible with the LiCoO.sub.2 and LiNiO.sub.2 carbon materials for which no air-stable higher lithium compositions are possible.
Another factor which further advanced development of rocking-chair cells was the use of a petroleum coke (a disordered graphite) as the carbon material in the negative electrode, which intercalates one lithium for twelve carbon atoms. This corresponds to about half the theoretical capacity of graphite in which the maximum lithium composition is LiC.sub.6. Recently, it was discovered that graphite can now be used with a true capacity corresponding to about 0.9 Li per 6 carbon atoms at cycling rates of one hour. See, Tarascon and Guyomard, Electro. Chem. Acta 1993, 38, 1221. The capacity is then almost doubled and the average voltage is reduced by about 0.3 volts compared to coke. Replacing coke by graphite in rocking-chair cells will result in an increase of specific energy by about 30%.
The use of lithium-ion cells in which both electrodes comprise intercalation materials such as lithiated manganese oxide and carbon are further described by Tarascon in U.S. Pat. No. 5,196,279. Guyomard and Tarascon, J. Electrochem Soc. Vol. 140, No. 11, November 1993, pp. 3071-81 further describes these rocking chair rechargeable batteries.
The separator or absorber-separator in the cell which is positioned in between and abutting the two electrodes presents some important considerations in construction of the cell. For example, the conductivity of the material in combination with the electrolyte should be sufficiently high so as not to impede the efficiency of the cell. Tsuchida et al., Electrochemica Acta, Vol. 28, 1983, No. 5, pp. 591-95 and No. 6, pp. 833-37 indicated that polyvinylidene fluoride compositions were capable of exhibiting ionic conductivity above about 10.sup.-5 S/cm only at elevated temperatures, reportedly due to the inability of the composition to remain homogeneous, i.e., free of salts and polymer crystallites, at or below room temperature. Enhanced ionic conductivity was obtained by Tsuchida and his coworkers, however, by incorporation of lithium salts and solvents that were compatible with both the polymer and salt components.
Accordingly, as can be seen from the foregoing references, the selection of the polymer employed in the rocking-chair cell has to be made to enhance ionic conductivity, and compatibility with the lithium salts and solvents employed as the electrolyte.
Lithium ion access to the surface of the active material is an important consideration in designing these types of cells. If the polymer coats the surface of the active materials, while still allowing lithium ion passage through it, then interaction of the electrode active materials with the electrolyte solution is minimized which is an additional benefit.
Gozdz et al., U.S. Pat. No. 5,296,318, describe a rocking-chair cell utilizing a polyvinylidene fluoride copolymer in the fabrication of the electrodes and the absorber-separator which contains the electrolyte. A rechargeable battery based on lithium intercalation compound electrodes and an interposed electrolyte flexible polymer containing a lithium salt dissolved in a polymer-compatible solvent is disclosed. The polymer comprises a copolymer of vinylidene fluoride and 8 to 25% of hexafluoropropylene.
In a specific example, the absorber-separator is based on an ethylene carbonate:propylene carbonate solution of LiPF.sub.6 in an 88:12 vinylidene fluoride hexafluoropropylene polymer whereas the positive electrode is based on this polymer in combination with SS carbon black LiMn.sub.2 O.sub.4 and LiPF.sub.6, further in combination with an aluminum metal lead attached to it. The negative electrode was based on the same polyvinylidene fluoride copolymer in combination with powdered petroleum coke, SS carbon black and the same LiPF.sub.6 electrolyte in ethylene carbonate:propylene carbonate solvent. The negative electrode in turn was connected to a copper metal lead.
Menassen et al. "A Polymer Chemist's View On Fuel Cell Electrodes," Proceeding Of The 34th International Power Source Symposium, Jun. 25-28, 1990, pp. 408-10, studied polyvinylidene fluoride binders in fuel cell electrodes and made a comparison to polytetrafluoroethylene (PTFE) sintered electrodes. It was noted that with polyvinylidene fluoride electrodes using the phase-in version method that contrary to the classical electrodes, where the active material resides in little islands between the sintered PTFE particles where the surface area of the carbon is of prime importance, much larger pores were obtained with polyvinylidene fluoride whose walls were made up of a composite polymer with carbon particles in a continuous polymeric matrix.
Accordingly, it would be an advantage to provide an electrochemical cell having electrodes and an absorber-separator that would make the utilization of the active material more efficient.
Additionally, it would be an advantage to provide segregation of an active composite polymer on the surface of active pores in a porous and especially a microporous electrode and/or absorber-separator which could allow for varying the amount of the polymer in the electrode or absorber-separator in order to obtain increased strength with minimum effect on cell performance.
These advantages would especially be valuable in very thin flexible secondary or rechargeable batteries that are currently being produced for consumer electronic products.
It would also be advantageous to provide a rechargeable cell, and especially a lithium rocking-chair type of battery that would readily lend itself to ease of fabrication from polymeric materials that could be either formed from solution, or by extrusion and which could be readily enveloped or packaged in an enclosure by fusion or heat lamination techniques.
Accordingly, the present invention is directed to an article of manufacture comprising an electrochemical cell that substantially obviates one or more of these and other problems due to limitations and disadvantages of the related art.